Their school doesn’t exist in actual Paris, like many other places in the series, including Marinette, Adrien, and Chloé’s homes, but we can conclude from a few scenes that it’s right across the Seine from the Notre Dame de Paris:
(Marinette’s house => Collège)
(Collège on the right.)
(Notre Dame de Paris on the left.)
There are a lot of other scenes that depict Marinette’s house right across the Seine from the Notre Dame as well. @paris-in-miraculous-ladybug might be able to give you some more information on this.